There must be no space between the -d and its
numeric arguments. If you put space there, the numeric arguments
might be interpreted as recipient addresses.

There is no way to isolate a single category and level. Each level
includes the output of all lower levels within a specified category.

The concept of debugging, versus other uses of -d,
is muddled in sendmail. Tracing, for example,
can be valuable for tuning a configuration file, yet such an activity
is not really debugging. We hope to make the distinction clear by
documenting only the "useful"
debugging switches, and omitting the true code-level debugging
switches from this chapter.

Because the -d command-line switch shows details
of the internals of sendmail, the developers of
sendmail consider that output to be unpublished
material. As a consequence, the details of debugging output
documented here might differ from what you see when running versions
above or below V8.12. You are strongly encouraged to avoid writing a
program to parse debugging output because such a program might become
obsolete with a future release of sendmail.